Supplying Generator Rotor Using Carbon Brushes


Thread Starter


why we give supply generator rotor DC directly through carbon brushes instead of exciter?

In our plant 275MW generators they are designed for rotor dc directly through carbon brushes. why we not use the exciter instead of carbon brushes?

DC isn't just something that comes from an outlet in the wall; it has to be "produced" and that implies some means of control. That is what the exciter does: it controls the production of DC from an AC source.

By the way, exciters are also called "AVRs" by many people (Automatic Voltage Regulators), a reference to controlling the DC field voltage/current in order to control the generator terminal voltage.

And since the generator rotor--and the generator field--is rotating (spinning) when the turbine is running how would you connect the exciter to the field leads (positive and negative) if you didn't use carbon brushes?

The carbon brushes allow current to flow from a brush holder which is connected directly to the DC output of the exciter (either via a bus bar or a very large cable) and apply it to the slip rings of the generator rotor which are connected to the positive and negative leads of the spinning generator rotor field.

There are brushless exciters, but they have their own pluses and minuses. A brushless exciter requires a rotating generator in addition to the generator rotor field and the output of the rotating generator is rectified through diodes also mounted on the rotating generator shaft which produces DC that is directly applied to the generator rotor field leads. And, in addition to this second rotating generator and diode wheel, there must be a stationary exciter regulator to apply DC to the stationary field of the rotating generator. So, there's a lot of extra rotating equipment, which increases the inertia of the generator rotor as well as maintenance and balance requirements.

Again, each type of system has its own pluses and minuses and there are even more than the ones mentioned above. But, for high current/voltage "transmission" from the exciter to the spinning generator field carbon brushes have proven to be more than adequate for more than a century of electric power generation.

Hope this helps!
Saud Aslam,

Thanks for your prompt reply.

Finally we conclude that direct brushes excitation is better than brushless excitation through exciter and there is no economical factor involve in big generators excitation through brushes.
Saud Aslam,

Carbon brushes are the best for transmitting high current from an exciter output to the rotating slip rings connected to the rotating generator field leads.

Brushless exciters have their place, but are typically not used on machines above approximately 100 MW. Why? I'm not sure; I just know I almost never seen brushless exciters on machines above approximately 100 MW. I think a lot of it has to do with the size of the rotating generator (mass; cost) and the size of the diodes which also must be mounted (and balanced) on the rotating diode wheel. Also, a very large rotating generator would make disassembly and removal of the main generator field very difficult unless some type of coupling was used between the generator rotor shaft and the exciter rotating generator shaft.

Since the advent of large, high-voltage, high-current rectifier bridges used to produce DC for synchronous generator fields the trend seems to have been to use carbon brushes to transmit the current to the rotating generator field. And, it's been done successfully for decades, and for decades before that when there were large rotating DC machines producing the power for synchronous generator fields.
CSA... are you sure you don't mean Brushless Excitation systems are used for very large alternators, and Slip-Ring excitation (Brush-type) are limited to smaller machines?

An advantage of a brushless exciter is better control of the generator under fault/transients conditions. The field current is directly controlled by scr/diodes/igbts and has minimal time lag. It is also easy to measure the field current directly. In a rotating exciter the stationary field voltage is controlled which the rotating exciter then produces/rectifies the rotor field dc current.

It's virtually impossible the measure the output of the rotating diode wheel of a brushless exciter. One can only measure the voltage applied to the stationary field of the rotating generator.

It's much easier to measure the DC current flowing through the conductors which are providing DC current to the brushes which are riding on the rotating slip rings and applying DC current to the rotating generator field.

And, one of the knocks against rotating DC exciters is that they can have some long lags in changing DC field current under transient conditions. They're not so great for large generators which may be expected to "clear" some types of faults; there are specific static rectifier bridges and schemes to provide the current necessary to do that without damaging the generator.

Smaller generators aren't usually asked to clear faults, and in an effort to reduce the maintenance (by not having to change brushes--which can be done when the generator is running, or maintain slip rings) and to reduce the costs of the generator exciter it's more common to use brushless exciters for smaller generators, though I've seen them on units rated as high as 85 MW.
I agree 100% I was trying to say the same thing. Must be my English.

> It's virtually impossible the measure the output of the rotating diode wheel
> of a brushless exciter. One can only measure the voltage applied to the stationary field.